The final defence witness in the Oscar Pistorius trial has told a court why the athlete did not flee from his house on the night he shot dead his girlfriend.

On the 37th day of the trial, the defence read from Pistorius' evidence - the part he was questioned about why he went towards danger after allegedly being startled by a noise.

Professor Wayne Derman, who worked with the South African Paralympic team, said the response was consistent with the "flight or fight" reflex, adding there was less ability to "think" after such instincts kicked in.

The physician said fleeing was not an option as the athlete has no lower legs.

The only other option was to fight and approach danger, he said.

Pistorius denies murdering 29-year-old model Reeva Steenkamp, claiming he shot her after mistaking her for an intruder at the Pretoria property.

Mr Derman said the defendant, who fired his pistol through a closed toilet door in his home, was hyper-sensitive to sound and has a heightened fight or flight response.

He told the court the 27-year-old South African is vulnerable and his fear of crime is exacerbated by his physical disability.

The athlete's lower legs were amputated as a baby because he was born without fibula bones.

Image:Pistorius has admitted killing girlfriend Reeva Steenkamp

Mr Derman, a professor of sport and exercise medicine at the University of Cape Town, also described the athlete as a "paradox".

He said there were "two" Oscars - one who won races with his hands aloft, standing tall on his prosthetic legs, the other disabled on stumps.

Video:Oscar 'Trained To React To Gunshot'

"It is different each day," he said, adding the contrast causes stress and anxiety for the double-amputee runner.

The defence wants to show Pistorius' alleged sense of vulnerability was a factor in the night-time shooting on Valentine's Day last year.

Under cross-examination, the prosecution claimed Mr Derman could not give objective evidence as an "expert" as it might harm his patient.

But Mr Derman rejected this and said he had backed up his points with science.

On Wednesday, the court heard the defendant has been suffering from post-traumatic stress disorder (PTSD) since shooting Ms Steenkamp and needs to continue being treated to avoid the risk of suicide.

A psychiatric report found Pistorius is not mentally ill but has been "severely traumatised" by the events of last year.

The athlete, who is on bail, faces between 25 years and life in prison if found guilty of premeditated murder.

But he could also be sentenced to a shorter prison term if convicted of murder without premeditation or negligent killing.